Method of toughening and modification of ceramic and ceramic product

ABSTRACT

When a sintered body of ceramic is shot-blasted at normal temperatures to plastically deform the crystal structure of the shot-blasted surface to apply residual stress and is heat-treated to recrystallize fine cracks, dislocated cells in the grain boundary are formed, crystals are finely divided, and the fracture toughness is significantly improved. When the sintered body of ceramic is a thin product, an effective toughening can be attained by shot blasting both the front and back sides. After heat treatment, mechanical strength is significantly improved by removing a part of the modified surface layer by an abrasion treatment.

TECHNICAL FIELD OF INVENTION

[0001] The present invention relates to a method of overcoming“fragility” in ceramic, which is the largest disadvantage for ceramic,and specifically relates to a method of significantly improving fracturetoughness. More specifically the present invention relates to a methoduseful for significantly improving the mechanical properties of heatengine parts of gas turbines, automobiles, etc., mechanical structuralelement parts such as springs and gears, and sensors, actuators, andmicromachine parts using a single-crystal material such as a siliconwafer, yttrium-aluminum-garnet (YAG), and sapphire, and apolycrystalline material such as barium titanate, and lead titanatezirconate (PZT), to increase their reliability, and relates to ceramicproducts produced by using the method.

DESCRIPTION OF RELATED ART

[0002] Generally ceramic members (products) are fragile and rigidmaterials. The fractural toughness of them is less than that of metals,and is a level of one-tenth to one-half of that of metals. Thereforeceramic is practically much lower in mechanical reliability comparedwith that of metals.

[0003] As the causes for the mechanical reliability of ceramic members(products) being quite low, the surface defect of them is exemplified.It is known that such a surface defect significantly affects mechanicalproperties. Usually a sintered body of ceramic has a surface defect, forvarious reasons (microcracks upon molding and drying, non-uniformshrinkage upon sintering, etc.). A surface, defect sometimes becomes theorigin of breakage. Thus it is desirable that the sintered body not haveany surface defect. However, at present it is difficult to form asintered body that has no surface defect. Also, it is very difficult tofind such a fine defect by a non-destructive inspection.

[0004] In these circumstances, to increase mechanical reliability,methods for removing any surface defect have been conventionallyadopted. That is, since upon preparing a sintered body of ceramic, by amethod of preparing it the volumetric shrinkage cannot be avoided, andthus it has been difficult to prepare a sintered body having excellentdimensional accuracy, a sintered body obtained after sintering ceramichas been subjected to an abrading processing to remove any surfacedefect to improve dimensional accuracy. However, fine cracking can occurupon mechanical processing such as abrading. Also, the methods formechanically removing a surface defect after sintering ceramic haveproblems such as that the processing cost is high.

[0005] As any other alternative method, a method of resintering asintered body, i.e., conducting a high-temperature treatment to remedy asurface defect, is known. Although this method is effective for asintered body of a non-oxide type material, on which an oxide layer iseasily formed, no sufficiently improving effect can be obtained for asintered body of an oxide-type material such as alumina or zirconia, onwhich an oxide layer can hardly be formed.

[0006] Also, a method was proposed of blast-processing with abrasivegrains in the heated state a sintered body to a temperature such thatthe fracture toughness of it increases. By that method, although thesurface is remedied no effect of modifying the fine structure of thecrystals is obtained. Also, since for that method a sintered body shouldbe blast-processed at such a high temperature it has disadvantages inthat it is a difficult operation, and the cost is high.

[0007] On the other hand, a trial has been made to improve fracturetoughness by improving the composition of a material. That is, a methodof selecting an optimum aid for sintering and closely controlling theamount of the aid to be added, and a method of adding seed crystals tothe material to control the fine structure of crystals, have beenproposed. However, no sintered body of ceramic having a fracturetoughness level of a metal (15 or more) has been found.

SUMMARY OF THE INVENTION

[0008] The present invention has been made in consideration of theproblems of the conventional art as stated above. The object of thepresent invention is to provide a method of toughening and modifyingceramic and ceramic products, by which method the properties andreliability of the products are improved and specifically by whichmethod the fracture toughness is greatly improved.

[0009] To achieve the above object (1) a method of toughening andmodifying ceramic has been invented, said method being characterized bycomprising a step of applying residual stress to a sintered body ofceramic by a shot-blasting treatment, a step of heat-treating thethus-obtained sintered body to recrystallize it, and a step of removinga part of the modified surface layer having dislocated cells obtained bythe preceding steps.

[0010] Also, a method of toughening and modifying ceramic is within thescope of the present invention, said method being characterized bycomprising (a) a step of applying a residual stress to a sintered bodyof ceramic by a shot-blasting treatment, (b) a step of heat-treating thesintered body treated by step (a) to recrystallize the part of ceramicnear fine cracks, and (c) a step of removing from said sintered bodyheat-treated by step (b) a part of the modified surface layer, saidlayer having dislocated cells in the sintered body of ceramic.

[0011] Also within the scope of the present invention is a method oftoughening and modifying ceramic characterized by comprising (a) a stepof applying residual stress to a sintered body of ceramic by ashot-blasting treatment, or introducing a cell defect such as adislocation by a plastic deformation, (b) a step of heat-treating thesintered body treated by step (a), to recrystallize the part of ceramicnear fine cracks to form a sub-boundary, and (c) a step of removing fromsaid sintered body having been heat-treated by step (b) a part of themodified surface layer of a sintered body of ceramic, said layer havingdislocated cells.

[0012] The inventors of the present invention have found that thecrystal structure of the surface of ceramic can be plastically deformedby shot blasting to apply residual stress to the structure. Also, theinventors have found that by heat-treating the sintered body of ceramicat a sintering temperature or less, the part of ceramic near fine crackscan be recrystallized to form dislocated cells at a grain boundary, tofinely divide the recrystallized structure to significantly improvefracture toughness. Also, the inventors have found that, when the cracksare remedied by heat-treating the plastically deformed structure, thepractical reliability of ceramic products can be significantlyincreased.

[0013] (2) Even if the sintered body is excessively plastically deformedby a shot-blasting treatment, it becomes easy to provide residual stressby blast-treating also the side opposite the side that has been blasted.Thus the method of claim 4 is within the scope of the present invention.In the blast-treating process both sides can be simultaneously blastedor each of both sides can be sequentially shot-blasted. Even if a partof a sintered body or a product is excessively plastically deformed,only that part may be blast-treated on both sides of that part.

[0014] (3) When the sintered body is a substrate, chip, thin sheet,foil, or flat plate, it is preferred that both sides of the sinteredbody be shot-blasted to apply residual stress to both sides.

[0015] (4) Also within the scope of the present invention is a method oftoughening and modifying ceramic of any of Nos. (1) to (3), said methodbeing characterized in that the temperature of the heat treatment is oneat which cracks are remedied. Herein the temperature at which cracks areremedied is referred to as being a temperature at which a dislocationintroduced near fine cracks is polygonized by a stabilizationrearrangement to form a sub-boundary, as well as atoms near fine cracksbeing diffused to eliminate and remedy at least part of the fine cracks.

[0016] (5) Also within the scope of the present invention is a method oftoughening and modifying ceramic of any of Nos. (1) to (4), in whichmethod the step of removing a part of the modified surface layer ischaracterized by abrasion-treating from the outer surface the modifiedpart wherein dislocated cells were obtained by a shot-blasting treatmentand heat treatment (to a depth of several micrometers to several hundredmicrometers). As abrasion treatment methods, barrel polishing, buffing,honing, and polishing are exemplified; however, any abrading method canbe used. An abrading method is selected depending on the kind ofproduct, etc.

[0017] (6) Ceramic products (including ceramic parts and members)produced by a method of toughening and modifying ceramic of any of Nos.(1) to (5) are within the scope in the present invention. Those productshave improved properties of some of fracture toughness, wear resistance,fatigue strength, resistance to thermal shock, and resistance to thermalfatigue.

[0018] (7) Also, within the scope of the present invention is a methodof toughening and modifying ceramic and ceramic products (includingceramic parts and members), said method being characterized bycomprising (a) a step of subjecting the sintered body of ceramic to ashot-blasting treatment with shot consisting of fine particles having anaverage particle diameter of 100 μm or less to apply residual stress tothe sintered body, and (b) a step of heat-treating the sintered bodytreated by step (a), to recrystallize the part of ceramic near finecracks.

[0019] Also, within the scope of the present invention is a method oftoughening and modifying ceramic and ceramic products (including ceramicparts and members), said method being characterized by comprising (a) astep of subjecting the sintered body of ceramic to a shot-blastingtreatment with shot consisting of fine particles having an averageparticle diameter of 100 μm or less to apply residual stress to thesintered body, or of plastically deforming the sintered body tointroduce a cell defect such as dislocation, and (b) a step ofheat-treating the sintered body treated by step (a) to recrystallize thepart of ceramic near fine cracks to form a sub-boundary.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The ceramic materials used in the present invention includesilicon nitride, silicon carbide, alumina ceramic; and single-crystalmaterials such as silicon, yttrium-aluminum-garnet, gallium arsenide,silicon germanium, sapphire, and a single-crystal alumina; andpolycrystalline materials such as barium titanate, lead titanatezirconate (PZT), and aluminum nitride. The ceramic products (includingceramic parts and members) of the present invention include mechanicalstructural element parts such as springs, knife edges, gears andbearings; heat engine parts such as automotive engine valves and bladesof gas turbines; and semiconductor elements, sensor elements,piezoelectric elements, micromachine parts, semiconductor substrates,substrates of electronic parts, etc., using a single crystal material orpolycrystalline material. The present invention is particularlyeffective in improving mechanical properties of thin parts such assensors and semiconductor substrates.

[0021] Step (a) is a step of a shot-blasting treatment of the sinteredbody of ceramic. The plastic deformation is in connection with shotblast conditions, particularly the hardness of shot, the particlediameter of shot, and projection energy such as impact speed, andsignificantly depends on kinetic energy at the impact of shot.

[0022] Also, when the conditions for the heat treatment of step (b), andconditions for the removal of a part of the modified surface layer ofthe product surface of step (c), e.g., conditions for the abradingtreatment, are properly selected, the strength and fracture toughness ofa sintered body can be significantly improved compared with those in thecase where the sintered body of ceramic is not treated by the method ofthe present invention. When fracture toughness is improved, asynergistic effect is obtained such that resistance to thermal shock,resistance to thermal fatigue, wear resistance properties, etc., aresecondarily significantly improved.

[0023] The major steps of the production process of ceramic products ofthe present invention are now shown: Adjustment of materialsmolding→processing→degreasing→sintering→(machining→) a shot-blastingtreatment→heat treatment→an abrading treatment→products. Herein,machining is optionally conducted.

[0024] The conditions for the shot-blasting treatment are selected suchthat stress is applied that is equivalent to or more than the stressused in the loaded state that is the same as the state in which theproduct is used, to set up residual compression stress to the surfacelayer. In the area to which residual compression stress is applied,remedying of cracks is accelerated and particularly fatigue strength isimproved.

[0025] When intended products are thin elements, a substrate, etc., theyare sometimes plastically deformed excessively so that residual stresscan hardly be applied, depending on the conditions for a blasttreatment. In such a case optimization of the conditions for the blasttreatment is of course important; however, residual stress may beeffectively applied simply by a blast treatment on both sides of asintered body. That is, even if a sintered body is excessively deformed,residual stress can be easily applied by blast-treating both sides ofthe body.

[0026] Although the proper temperatures for the heat-treatment arevaried depending on the kinds of materials, they are those at whichrecrystallization starts (e.g., sintering temperatures, temperaturesthat are one half of or more than the temperatures at which coagulationstarts), and specifically suitable are those in the range of 1,000° C.to 1,600° C. The heat treatment should be conducted in atmosphere, undervacuum, or in an inert gas. Particularly when it is conducted inatmosphere the strength-improving effect is high.

[0027] When a heat treatment is conducted after shot blasting, thesurface layer is plastically deformed to form a dislocated cellstructure to form a surface layer comprised of fine crystals by a staticrecrystallization. It is recognized that such a deforming of a surfacelayer improves not only fracture toughness and strength but alsomechanical properties such as fatigue strength and wear resistance,resistance to thermal shock, and resistance to thermal fatigue. Adislocated cell is formed primarily by a plastic deformation andsecondarily by a heat treatment.

[0028] The effects obtained by the shot blasting and heat treatment areseen in the depth direction of only dozens of μm to several hundred μm.However, when the process of the present invention is applied to a thinsubstrate having a thickness of several hundred μm, significant effectscan be obtained.

[0029] Since the blasting and heat treatments form concave and convexscars from traces of strikes and an oxide layer on a surface, anabrading treatment is conducted in order to remove a part of themodified surface layer. The thickness of the modified surface layer inwhich a dislocated cell is formed is from dozens of μm to severalhundred μm. Accordingly the thickness to be removed for a rough surfacelayer and oxide layer should have an optimum value. This is because ifthe modified surface layer were to be entirely removed the tougheningeffect would be lost. The thickness of the layer to be removed ispreferably to be at a level such that the irregular surface resultingfrom the blast treatment becomes even, that is, to be within a level ofseveral μm to several hundred μm, more preferably about 5 μm to about 20μm. However, when fine particles of 100 μm or less are used as shot, atreated surface without concave and convex scars from traces of strikesis obtained. Further, when a material is used in which production of anoxide layer in the subsequent heat treatment does not affect it or in acondition under which no oxide layer is formed, then even if an abradingtreatment step is omitted a toughened and modified sintered body can beobtained.

[0030] Fracture toughness means resistance to elongation of cracks. Asmeasures of fracture toughness, physical quantities are used, such asfracture toughness value (K1C), hardness, and Young's modulus. Hardness,which can be measured most easily, is preferably used as the measure.

[0031] Shot used for the shot-blasting treatment of step (a) include,for example, mullite, SiC, carbide alumina, zirconia, and glass. Eachparticle of shot preferably should take the shape of a sphere as much aspossible since then it does not undesirably damage a substance to betreated. A shot is selected and used depending on the materials andhardness of the sintered body of ceramic. To impact shot on a surface ofsintered body, any of shot blasting, air blasting, and liquid honing canbe utilized.

[0032] In an air-blasting treatment, air or an inert gas is used as amedium. In a liquid-honing treatment, water is usually used as a medium.

[0033] The conditions for a shot-blasting treatment depend on thehardness of the shot, particle diameter, impact speed, etc., andsignificantly depend on the kinetic energy at the time of the impact ofparticles. The kinetic energy that particles have is converted toplastic deformation, elastic deformation, and fracture energy of thesubstance to be treated or particles, and frictional energy between asubstance to be treated and particles, etc. In any blast treatmentmethod, the conditions should be set such that the kinetic energyapplied to the substance to be treated is the maximum. For that reasonit is important to project particles perpendicularly to the substance tobe treated. Also, a preferable shot-blasting treatment should reduceirregularities on a surface layer, i.e., roughness of the surface, andincrease the thickness of the plastically deformed layer (residualstress-applied layer), and for such a preferable shot-blasting treatmentthe optimum conditions for each material should be selected.

EFFECTS OF THE INVENTION

[0034] According to the present invention mentioned in (1) above,toughening ceramic with simplicity and excellent economy compared withconventional methods is realized. That is, toughening ceramic can berealized by a step of shot blasting the sintered body of ceramic to makeit plastically deformed so as to apply residual stress to the surfacestructure of the sintered body, a step of heat treatment, and a step ofan abrading treatment so as to flatten the surface in which fine cracksare recrystallized.

[0035] According to the present invention mentioned in (2) above, byshot blasting both sides appropriate residual stress can be applied tothe sintered body, as well as modifying the deformation of the sinteredbody of ceramic.

[0036] As in the present invention mentioned in (3) above, the presentinvention is suitable for improving the toughness of ceramic parts usedfor functional materials for a substrate of thin film, sensors, etc.

[0037] Also, according to the present invention mentioned in (4) above,since a heat treatment is conducted at a temperature at which cracks areremedied, the heat treatment of this invention allows the part ofceramic near fine cracks to be recrystallized, and thus it reduces thefine cracks in the surface structure. Thereby the cracks are remediedand thus the sintered body is toughened.

[0038] Since, according to the present invention mentioned in (5) aboveon the modified surface having dislocated cells that have been formed bysteps (a) and (b) concave and convex scars of traces from strikes and anoxide layer are abraded, the mechanical properties of ceramic productsare improved.

[0039] Also, according to the present invention mentioned in (7), whenas a particle for a shot-blasting treatment, fine particles, preferablyparticles having an average particle diameter of 100 μm or less, areused, a surface without concave and convex scars of traces from strikescan be obtained, and thus the removal of a part of the modified surfacelayer is unnecessary.

[0040] As is clear from the above, since, according to the presentinvention, wherein a process of heat-treatment is carried out aftershot-blasting the sintered body of ceramic at normal temperatures,recrystallizing the part of the ceramic near fine cracks occurs by theheat treatment after plastically deforming the crystal structure of thesurface of the sintered body to apply residual stress, dislocated cellsare formed in the grain boundary, and the sizes of the crystals areminimized to significantly improve fracture toughness. By removing apart of the modified surface layer after the heat treatment, themechanical strength of the sintered body is significantly improved.

EXAMPLES Evaluation Methods

[0041] First, the evaluation methods in the Examples will bespecifically explained.

[0042] Following the Japan Industrial Standards, test specimen materialswere subjected to a mechanical processing into a size of 3 mm×4 mm×40 mmto make test pieces for bending tests. Bending tests were conductedfollowing JIS-R-1601 (a three-point bending test). Also, fracturetoughness values were determined by measuring the length of a crack madeby a Vickers indenter with a Vickers hardness tester varying indentationload conditions. That is, following JIS-R-1607, fracture toughness tests(IF method) were conducted to determine them.

[0043] The optimum value of the indentation load condition was selecteddepending on the kind of material and set to be 1 kgf, 2 kgf, 5 kgf, or10 kgf. The time for retaining a load was set at 20 seconds. Theconditions were set such that the temperature of the heat treatment was1,200° C. to 1,400° C., retention time one hour, and the condition ofthe atmosphere was in atmosphere or under vacuum and the rate oftemperature rise was 10° C./minute. When silicon nitride is heated to1,000° C. or more in atmosphere, oxidation proceeds; however, theoxidation level does not affect the properties of materials providedthat the temperature of the heating is 1,300° C. or less. When anyproblem occurs, a heating treatment may be conducted in a nitrogen gasor inert gas. The evaluations of fracture toughness were conducted usingas test pieces for evaluation those that were mirror-polished such thatthe surface roughness was 0.2S or less, by abrading the surfaces of thetest pieces sequentially with diamond-abrasive papers of #600, #1000 and#3000. Plastic deformation was determined by measuring by a lasermicroscope the maximum deformation of the test pieces in the convexstate.

Test Specimens

[0044] The test specimens below were used in each Example.

[0045] Test specimen No. 1: an alumina ceramic material (alumina purity:92%)

[0046] Test specimen No. 2: an alumina ceramic material (alumina purity:99.5%)

[0047] Test specimen No. 3: an alumina ceramic material (alumina purity:99.99%)

[0048] Test specimen No. 4: a silicon nitride ceramic material (siliconnitride (Refercerm SN1, prepared by Japan Fine Ceramic Center: aid forsintering (Ce+MgO) type)

[0049] Test specimen No. 5: silicon nitride (prepared by SINTO V-CERAX,LTD.: aid for sintering (Y203+Al203) type)

[0050] Test specimen No. 6: an aluminum nitride material (aid forsintering (Y203) type)

[0051] Test specimen No. 7: silicon wafer

[0052] Test specimen No. 8: single crystal alumina

Example 1

[0053] Conditions for shot-blasting treatments and toughening effects

[0054] Using Test specimen No. 2 (alumina: 99.5%) and Test specimen No.5 (silicon carbide), test pieces (size: 5 mm width×80 mm length×0.3 mmthickness) were prepared. When one side of a surface of 5 mm×80 mmsurface was blast-treated from the vertical direction to that side (inthe direction of thickness) and when both sides of those surfaces of 5mm×80 mm surfaces were blast-treated, plastic deformation and fracturetoughness levels obtained after the test pieces were heat-treated werecompared. Their results are shown in Table 1. As conditions for the shotblasting, atmospheric condition, and as the shots, two types ofparticles, mullite particles (Cerabeads 60, #1700: prepared by NaigaiCeramics Co. Ltd.) and carbide particles (ST-160, prepared bySINTOBRATOR, LTD.), were selected. For mullite particles the treatmentwas conducted using pressure shot-blasting equipment (produced bySINTOBRATOR, LTD.) under conditions such that the pressure for theprojection was 1.0 kg/cm², and the nozzle diameter for the projectionwas 6 φ. For carbide particles the treatment was conducted using gravityshot-blasting equipment (produced by SINTOBRATOR, LTD.) under conditionssuch that the pressure of the projection was 2.0 kg/cm², and the nozzlediameter for projection was 8 φ. The blasting times were 30 seconds to60 seconds. Heat treatments were conducted in atmosphere at 1,300° C.for one hour. TABLE 1 Results of Comparison of Improved Effects inFracture Toughness under Shot-blasting Treatment Conditions PlasticDeformation Plastic Improved Effect in Test Kind of Shot Surface beforeHeat Deformation after Fracture Fracture Toughness Specimen ShotDuration Roughness Treatment Heat Treatment Toughness Value (Comparisonof Effect No. Material (Seconds) Shot Side (Rmax · μm) *1 (mm) *1 (mm)KIC (Mpa · ^(½)) with Untreated) 5 Untreated 0.2 No Deformation NoDeformation 6.48 1.00 Cerabeads 30 One Side 20 0.51 5.03 6.95 1.07Cerabeads 60 One Side 25 0.75 7.63 7.18 1.11 Cerabeads 90 One Side 300.73 7.34 7.24 1.12 Super 30 One Side 5 0.83 9.24 7.29 1.12 hard shotSuper 60 One Side 5 1.24 10.32 7.31 1.13 hard shot Cerabeads 30 BothSides 20 No Deformation No Deformation 7.58 1.16 Cerabeads 60 Both Sides25 No Deformation No Deformation 8.37 1.29 Cerabeads 80 Both Sides 30 NoDeformation No Deformation 8.25 1.27 Super 30 Both Sides 5 NoDeformation No Deformation 9.34 1.44 hard shot Super 60 Both Side 5 NoDeformation No Deformation 10.58  1.63 hard shot 2 Untreated 0.2 NoDeformation No Deformation 3.02 1.00 Cerabeads 30 One Side 13 0.21 2.433.19 1.06 Cerabeads 90 One Side 17 0.27 2.57 4.55 1.51 Cerabeads 30 BothSides 13 No Deformation No Deformation 4.88 1.62 Cerabeads 90 Both Sides17 No Deformation No Deformation 5.40 1.79

[0055] From the results of Table 1 it is seen that toughening effectssignificantly vary depending on the material of the ceramic and thematerial of the shot. Also it is seen that plastic deformationssignificantly vary depending on the conditions for blast treatments.Also, it is seen that when blast treatments are applied to both sideshigher effects are obtained. It can be recognized that when a blasttreatment is conducted on only one side, lower effects are obtained,while when it is conducted on both sides, higher effects are obtained.It can be recognized that this is because when both sides wereblast-treated residual stress is easily applied while when only one sideis blast-treated residual stress hardly remains due to plasticdeformation.

Example 2

[0056] Conditions for a heat treatment of polycrystalline materials andthe improved effects in fracture toughness

[0057] Six kinds of test specimens, Nos. 1-6, were evaluated. Followingthe Japan Industrial Standards, test specimens were subjected to amechanical processing into a size of 3 mm×4 mm×40 mm to make test piecesfor a bending test. Only one side of the surfaces, 4 mm×40 mm, wasblast-treated in the direction of the thickness of the test pieces (3mm). Thereafter heat treatments were conducted in atmosphere followed byan abrading treatment, to have samples be evaluated. The results areshown in Table 2. TABLE 2 Improved Effects in Fracture Toughness of EachPolycrystalline Material Improved Heat Effect in Treatment FractureFracture Test Blast Treatment Condition Toughness Toughness Speci-Condition Treatment KIC (Comparison men Shot Material; Shot Temperature(MPa · of Effect with No. Duration (° C.) m^(½)) Untreated 1 UntreatedUntreated 2.46 1.00 Untreated 1300 3.65 1.18 Cerabeads; 30 sec. 13004.55 1.85 Cerabeads; 30 sec. 1300 3.34 1.36 Super Hard; 50 sec. 13004.00 1.64 Super Hard; 50 sec. 1400 4.63 1.88 2 Untreated Untreated 3.021.00 Untreated 1300 3.34 1.11 Cerabeads; 30 sec. 1300 3.19 1.06Cerabeads; 60 sec. 1300 4.55 1.61 Super Hard; 60 sec. 1300 5.45 1.80Super Hard; 60 sec. 1400 5.65 1.87 3 Untreated Untreated 3.02 1.00Untreated 1200 3.19 1.06 Cerabeads; 60 sec. 1200 4.34 1.44 4 UntreatedUntreated 5.93 1.00 Untreated 1200 9.67 1.63 Untreated 1300 6.69 1.13Cerabeads; 30 sec. 1300 9.51 1.60 Cerabeads; 60 sec. 1200 9.14 1.54Cerabeads; 90 sec. 1300 15.16 2.56 Super Hard; 60 sec. 1300 10.65 1.80 5Untreated Untreated 6.49 1.00 Untreated 1200 8.15 1.26 Untreated 13006.02 0.93 Cerabeas; 30 sec. 1200 7.62 1.17 Super Hard; 60 sec. 1300 8.081.24 Super Hard; 60 sec. 1400 10.54 1.62 6 Untreated Untreated 2.16 1.00Cerabeas; 60 sec. 1200 2.84 1.32

[0058] It is seen that at temperatures of the heat treatment of 1,200°C. or more, fracture toughness is significantly higher than for anuntreated sintered body. Particularly in the case of silicon nitride(Test specimen No. 4), the fracture toughness (K1C) was 15 MPa·m^(1/2)(when Cerabeads were used for 60 seconds, and at 1,300° C.) and 2.6times the fracture toughness of an untreated sintered body. Thatfracture toughness level corresponds to that for cast iron, which noceramic has ever before attained.

Example 3

[0059] Conditions for heat treatment in single crystal materials andimproved effects in fracture toughness

[0060] A silicon wafer (Test specimen No. 7) and single crystal alumina(Test specimen No. 8), both of which were single crystal materials, wereused. The size for test specimens was 10 mm×10 mm×1.0. One side of thesurfaces, of 10 mm×100 mm, was blast-treated in the direction of thethickness. Heat treatments were conducted under vacuum for the siliconwafer and in atmosphere for the single crystal alumina, followed by anabrading treatment to make test pieces of which the fracture toughnesseswere evaluated. The results are shown in Table 3. TABLE 3 ImprovedEffects in Fracture Toughness of Single Crystal Materials Improved HeatEffect in Treatment Fracture Fractures Test Blast Treatment ConditionToughness Toughness Speci- Condition Treatment KIC (Comparison men ShotMaterial; Temperature (MPa · of Effect with No. Shot Duration (° C.)m^(½)) Untreated 7 Untreated Untreated 0.91 1.00 Untreated 1200 0.981.08 Cerabeads; 5 sec. 1200 1.24 1.36 Cerabeads; 10 sec. 1200 1.09 1.20Cerabeads; 15 sec. 1200 1.03 1.13 Cerabeads; 15 sec. 1300 1.53 1.68Cerabeads; 15 sec. 1400 1.89 2.07 8 Untreated Untreated 0.91 1.00Untreated 1300 1.37 1.50 Cerabeads; 30 sec. 1300 3.70 4.05 Cerabeads; 30sec. 1400 3.65 4.00

[0061] The fracture toughness obtained by a heat treatment at 1,200° C.or more is recognized as being significantly high compared with anuntreated sintered body.

Example 4

[0062] Improved effects in strength obtained by an abrading treatment

[0063] The bending strengths were compared for samples to which after aheat treatment no abrading treatment was applied and samples to whichabrading treatments were applied after a heat treatment. The results areshown in Table 4. TABLE 4 Improved Effects in Strength by AbradingTreatment after Blast and Heat Treatments Improved Effect in HeatFlexural Treatment Strength Blast Treatment Condition ComparisonCondition Treatment Grinding Flexural of Strength Test Specimen ShotMaterial; Temperature Treatment Strength with No. Shot Duration (° C.)Yes or No MPa Untreated 1 Untreated Untreated No 362 1.00 Cerabeads;1300 No 353 0.98 60 sec. Cerabeads; 1300 Yes 384 1.06 60 sec. 2Untreated Untreated No 406 1.00 Cerabeads; 1300 No 372 0.92 60 sec.Cerabeads; 1300 Yes 449 1.11 60 sec. 4 Untreated Untreated No 871 1.00Cerabeads; 1300 No 573 0.77 60 sec. Cerabeads; 1300 Yes 896 1.03 60 sec.5 Untreated Untreated No 751 1.00 Cerabeads; 1300 No 727 0.97 60 sec.Cerabeads; 1300 Yes 810 1.08 60 sec. 6 Untreated Untreated No 386 1.00Cerabeads; 1300 No 344 0.89 60 sec. Cerabeads; 1300 Yes 395 1.02 60 sec.

[0064] From Table 4 it is seen that although the samples to which ablast treatment and heat treatment had been applied have lower strengthscompared with samples to which no treatment had been conducted(untreated), when an abrading treatment is conducted for the formersamples the strengths recover. Before the abrading treatment, shot isprojected through a surface and into the sintered body, and scars wereobserved. The scars significantly affect bending strengths, and thestrengths are significantly improved by the abrading treatment. Theprojecting-through of shot may be avoided by the projection of fine shotbefore heat treatment.

Example 5

[0065] Optimum values in depth direction and the width removed byabrading for toughening and modifying effects

[0066] A modified surface layer is formed on a surface of the sinteredbody of ceramic by a shot-blasting treatment followed by a heattreatment. When the modified surface layer is abraded, bending strengthincreases; however, if the width removed by the abrading is large, theeffects obtained by the above treatments are lost. Accordingly fracturetoughness values were determined for samples in which the widths removedby abrading are changed and the relationship was examined between theabraded depths and the modified effects. The results are shown in Table5. TABLE 5 Effects in Depth Direction in Toughening and ModifyingEffects Blast Improved Treatment Heat Effect in Condition TreatmentFlacture Shot Condition Width Fracture Toughness Material; TreatmentRemoved by Toughness (Comparison Test Specimen Shot Temperature GrindingKIC (MPa · of Effect with No. Duration (° C.) (μm) m^(½)) Untreated) 5Untreated Untreated 6.49 1.00 Cerabeads; 1300 5 8.53 1.31 60 sec.Cerabeads; 1300 10 8.32 1.28 60 sec. Cerabeads; 1300 15 8.45 1.30 60sec. Cerabeads; 1300 20 8.08 1.24 60 sec. Cerabeads; 1300 25 6.55 1.0160 sec. 2 Untreated Untreated 3.02 1.00 Cerabeads; 1300 5 5.45 1.80 60sec. Cerabeads; 1300 10 6.42 1.79 60 sec. Cerabeads; 1300 15 6.48 1.8160 sec. Cerabeads; 1300 20 3.55 1.18 60 sec.

[0067] When the width removed by abrading increases to some extent, afracture toughness level does not change, but rather remains the same asthe level obtained where the sintered body is untreated. From thatresult it is found that there is an optimum level of the width to beremoved by abrading. For example, for the conditions for the treatmentsshown in Table 5, for test specimen No. 5, the removal in a width of 5μm to 20 μm is optimum. Also, for test specimen No. 2, the removal of awidth of 5 μm to 20 μm is optimum.

Example 6

[0068] Effects obtained by using fine particles as shot

[0069] Test pieces for a bending test by JIS were made. Blast treatmentswere conducted for these test pieces using zirconia beads having anaverage particle diameter of 50 μm followed by a heat treatment at 1300°C. for one hour. The surface roughness of the surface treated before andafter a blast treatment, and the bending strengths obtained with anabrading treatment or without an abrading treatment, were determined.The results are shown in Table 6. TABLE 6 Effects on Shot MaterialHaving Fine Particle Sizes (Omission of Process of Removing ModifiedSurface Layer by Grinding) Heat Improved Effect Treatment Surface InFlexural Blast Treatment Condition Roughness Width Strength ConditionTreatment R max; μm Removed by Flexural (Comparison of Test SpecimenShot Material; Temperature Before After Grinding Strength Strength withNo. Shot Duration (° C.) Blasting Blasting (μm) MPa Untreated) 5Untreated Untreated 0.20 871 1.00 Zirconia Beads; In Nitrogen 0.20 0.220 889 1.02 60 sec. Gas; 1300 Zirconia Beads; In Nitrogen 0.20 0.22 5 8931.03 60 sec. Gas; 1300 Zirconia Beads; In Nitrogen 0.20 0.23 10 887 1.0260 sec. Gas; 1300 2 Untreated Untreated 0.20 406 1.00 Zirconia Beads;1300 0.20 0.25 0 435 1.07 60 sec. Zirconia Beads; 1300 0.20 0.24 5 4411.09 60 sec. Zirconia Beads; 1300 0.20 0.25 10 439 1.08 60 sec.

[0070] No significant difference was found in the surface roughnessbefore and after a shot blasting with shot having fine particlediameters. That fact shows a surface without concave and convex scarsfrom traces from strikes was obtained. Also, from the results of thedetermination of bending strength it was found that in such ashot-blasting condition, without an abrading treatment strength isimproved, and thus the process of the abrading treatment can be omitted.No significant difference was found in the fracture toughness levels K1Cwith or, without an abrading treatment, although Table 6 does not referto this. The fracture toughness levels of the treated products wereimproved compared with those of the untreated products.

1. A method of toughening and modifying ceramic characterized bycomprising a step of applying residual stress to a sintered body ofceramic by a shot-blasting treatment, a step of heat-treating theresulting sintered body to recrystallize it, and a step of removing apart of the modified surface layer having dislocated cells resultingfrom the two preceding processes.
 2. A method of toughening andmodifying ceramic characterized by comprising (a) a step of applyingresidual stress to a sintered body of ceramic by a shot-blastingtreatment, (b) a step of heat-treating the sintered body treated by step(a) to recrystallize ceramic near fine cracks, and (c) a step ofremoving a part of the modified surface layer having dislocated cells ofthe sintered body of ceramic heat-treated by step (b).
 3. A method oftoughening and modifying ceramic characterized by comprising (a) a stepof applying residual stress to a sintered body of ceramic by ashot-blasting treatment, or of introducing a cell defect such as adislocation by a plastic deformation, (b) a step of heat-treating thesintered body treated by step (a) to recrystallize the part of ceramicnear fine cracks to form a sub-boundary, and (c) a step of removing apart of the modified surface layer having dislocated cells of thesintered body of ceramic heat-treated by step (b).
 4. A method oftoughening and modifying ceramic of any of claims 1-3 wherein ashot-blasting treatment is applied to opposing sides of the sinteredbody.
 5. A method of toughening and modifying ceramic of any of claims1-4 wherein the sintered body is a substrate, tip, thin film, foil, orflat plate.
 6. A method of toughening and modifying ceramic of any ofclaims 1-5 wherein the temperature of the heat treatment is thetemperature at which cracks are remedied.
 7. A method of toughening andmodifying ceramic of any of claims 1-6 wherein step (c) is a step ofremoving by abrading from the outer surface the modified surface layerin a depth of several μm to several hundred μm.
 8. Ceramic productsproduced by using the method of toughening and modifying ceramic of anyof claims 1-7.
 9. Ceramic products of claim 8 wherein some properties offracture toughness, wear resistance, fatigue strength, resistance tothermal shock, and resistance to thermal fatigue are improved comparedwith an untreated sintered body of ceramic.
 10. Ceramic productscharacterized by being obtained by producing them via a step of applyinga residual stress to the sintered body of ceramic and a step of removinga part of a modified surface layer having dislocated cells obtained byrecrystallizing ceramic by heating it.
 11. A method of toughening andmodifying ceramic characterized by comprising (a) a step of applyingresidual stress to the sintered body of ceramic by a shot-blastingtreatment with fine particles having an average particle diameter of 100μm or less, and (b) a step of heat treating the sintered body treated bystep (a) to recrystallize the part of ceramic near fine cracks.
 12. Amethod of toughening and modifying ceramic characterized by comprising(a) a step of applying residual stress to the sintered body of ceramicby a shot-blasting treatment with fine particles having an averageparticle diameter of 100 μm or less, or of introducing a cell defectsuch as dislocation by a plastic deformation, and (b) a step ofheat-treating the sintered body treated by step (a) to recrystallize thepart of ceramic near fine cracks to form a sub-boundary.
 13. A method oftoughening and modifying ceramic of claim 11 or claim 12 wherein theshot-blasting treatment is applied to opposing sides of the sinteredbody.
 14. A method of toughening and modifying ceramic of any of claims11-13 wherein the sintered body is a substrate, tip, thin film, foil, orflat plate.
 15. A method of toughening and modifying ceramic of any ofclaims 11-14 wherein the temperature of the heat treatment is thetemperature at which cracks are remedied.
 16. Ceramic products producedby using the method of toughening and modifying ceramic of any of claims11-15.
 17. Ceramic products of claim 16 wherein some properties offracture toughness, wear resistance, fatigue strength, resistance tothermal shock, and resistance to thermal fatigue are improved comparedwith an untreated sintered body of ceramic.